Multiwavelength SERS of Magneto-Plasmonic Nanoparticles Obtained by Combined Laser Ablation and Solvothermal Methods

The present study introduces a novel method for the synthesis of magneto-plasmonic nanoparticles (MPNPs) with enhanced functionality for surface-enhanced Raman scattering (SERS) applications. By employing pulsed laser ablation in liquid (PLAL) to synthesize plasmonic nanoparticles and wet chemistry to synthesize magnetic nanoparticles, we successfully fabricated chemically pure hybrid Fe3O4@Au and Fe3O4@Ag nanoparticles. We demonstrated a straightforward approach of an electrostatic attachment of the plasmonic and magnetic parts using positively charged polyethylenimine. The MPNPs displayed high SERS sensitivity and reproducibility, and the magnetic part allowed for the controlled separation of the nanoparticles from the reaction mixture, their subsequent concentration, and their precise deposition onto a specified surface area. Additionally, we fabricated alloy based MPNPs from AgxAu100-x (x = 50 and 80 wt %) targets with distinct localized surface plasmon resonance (LSPR) wavelengths. The compositions, morphologies, and optical properties of the nanoparticles were characterized by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-vis spectroscopy, and multiwavelength Raman spectroscopy. A standard SERS marker, 4-mercaptobenzoic acid (4-MBA), validated the enhancement properties of the MPNPs and found an enhancement factor of 2 × 108 for the Fe3O4@Ag nanoparticles at 633 nm excitation. Lastly, we applied MPNP-enhanced Raman spectroscopy for the analysis of the biologically relevant molecule adenine and found a limit of detection of 10-7 M at 785 nm excitation. The integration of PLAL and wet chemical methods enabled the relatively fast and cost-effective production of MPNPs characterized by high SERS sensitivity and signal reproducibility that are required in various fields, including biomedicine, food safety, materials science, security, and defense.

An anisotropic nanobox based core-shell-satellite nanoassembly of multiple SERS enhancement with heterogeneous interface for stroke marker determination

Herein, A novel gold-silver alloy nanobox (AuAgNB)@SiO₂-gold nanosphere (AuNP) nanoassembly based on core-shell-satellite structure is fabricated and applied to the surface-enhanced Raman scattering (SERS) detection of S100 calcium-binding protein B protein (S100B). It contains an anisotropic hollow porous AuAgNB core with rough surface, an ultrathin silica interlayer labeled with reporter molecules, and AuNP satellites. The nanoassemblies were systematically optimized by tuning the reporter molecules concentration, silica layer thickness, AuAgNB size, and the size and number of AuNP satellite size. Remarkably, AuNP satellites are adjacent to AuAgNB@SiO₂, developing AuAg-SiO₂-Au heterogeneous interface. With the strong plasmon coupling between AuAgNB and AuNP satellites, chemical enhancement from heterogeneous interface, and the tip "hot spots" of AuAgNB, the SERS activity of the nanoassemblies was multiply enhanced. Additionally, the stability of nanostructure and Raman signal was significantly improved by the silica interlayer and AuNP satellites. Eventually, the nanoassemblies were applied for S100B detection. It demonstrated satisfactory sensitivity and reproducibility with a wide detection range of 10 fg/mL-10 ng/mL and a limit of detection (LOD) of 1.7 fg/mL. This work based on the AuAgNB@SiO₂-AuNP nanoassemblies with multiple SERS enhancements and favorable stability demonstrates the promising application in stroke diagnosis.

Microfluidic In Situ Patterning of Silver Nanoparticles for Surface-Enhanced Raman Spectroscopic Sensing of Biomolecules

This work integrates the advantages of microfluidic devices, nanoparticle synthesis, and on-chip sensing of biomolecules. The concept of microreactors brings new opportunities in chemical synthesis, especially for metallic nanoparticles favorable in surface-enhanced Raman spectroscopy (SERS) for high-resolution and low-limit detection of biomolecules. However, still missing is our understanding of reactions at the microscale and how microsystems can be exploited in biosensing applications via precise control of nanomaterial synthesis. We investigate how microfluidic geometry affects nanoparticle patterning for high-resolution SERS-based sensing and propose a spiral-shaped microchannel that can achieve enhanced mixing, rapid reaction at room temperature, and uniform in situ patterning. The roles of channel geometry as the key parameter on patterning have been studied systematically to provide insight into the rational design of continuous microfluidic systems for SERS applications. We also demonstrate potential applications of this integrated system in label-free on-chip detection of 1 pM rhodamine B (enhancement factor, ∼4.3 × 10¹¹) and a 1 nM 41-base single-stranded deoxyribonucleic acid (DNA) sequence (enhancement factor, ∼1.5 × 10⁸). Our ready-to-use multifunctional system provides an alternative strategy for the facile fabrication of SERS-active substrates and promotes system integration, miniaturization, and on-site biological applications.

Recent progress of microfluidics in surface-enhanced Raman spectroscopic analysis

Surface-enhanced Raman spectroscopy is a significant analytical tool capable of fingerprint identification of molecule in a rapid and ultrasensitive manner. However, it is still hard to meet the requirements of practical sample analysis. The introduction of microfluidics can effectively enhance the performance of surface-enhanced Raman spectroscopy in complex sample analysis including reproducibility, selectivity, sensitivity, and speed. This review summarizes the recent progress of microfluidics in surface-enhanced Raman spectroscopic analysis through four combination approaches. First, microfluidic synthetic techniques offer uniform nano-/microparticle fabrication approaches for reproductive surface-enhanced Raman spectroscopic analysis. Second, the integration of microchip and surface-enhanced Raman spectroscopic substrate provides advanced devices for sensitive and efficient detection. Third, microfluidic sample preparations enable rapid separation and preconcentration of analyte prior to surface-enhanced Raman spectroscopic detection. Fourth, highly integrated microfluidic devices can be employed to realize multistep surface-enhanced Raman spectroscopic analysis containing material fabrication, sample preparation, and detection processes. Furthermore, the challenges and outlooks of the application of microfluidics in surface-enhanced Raman spectroscopic analysis are discussed.

Microfluidics and Surface-Enhanced Raman Spectroscopy: A Perfect Match for New Analytical Tools

In this perspective article, we emphasize the combination of Surface-Enhanced Raman Spectroscopy (SERS) and Microfluidic devices. SERS approaches have been widely studied and used for multiple applications including trace molecules detection, in situ analysis of biological samples and monitoring or, all of them with good results, however still with limitations of the technique, for example regarding with improved precision and reproducibility. These implications can be overcome by microfluidic approaches. The resulting coupling Microfluidics - SERS (MF-SERS) has recently gained increasing attention by creating thundering opportunities for the analytical field. For this purpose, we introduce some of the strategies developed to implement SERS within microfluidic reactor along with a brief overview of the most recent MF-SERS applications for biology, health and environmental concerns. Eventually, we will discuss future research opportunities of such systems.

Inline integration of offset MMF-capillary-MMF structure as a portable and compact fiber-optic surface-enhanced Raman scattering microfluidic chip

A novel fiber-optic surface-enhanced Raman scattering (SERS) microfluidic chip integrated with an embedded Raman probe is presented and demonstrated. The Raman probe consists of the offset-multimode-fiber (MMF)-capillary-MMF (OMCM) structure and SERS substrate. The probe is embedded in the microfluidic channel to form a compact and portable chip. The chip is employed with a fiber coupler in an all-fiber detection system, which has a good stability compared to the free space focusing by the conventional confocal Raman microscope. The excitation light transmits from both ends of the OMCM probe into the capillary, and the generated Raman scattered signals are collected by two MMFs simultaneously. Experimental results of the Rhodamine 6G (R6G) detection show that the Raman signal intensity increases in a linear pattern at ∼1509  cm^-1 with the increase of R6G concentrations. This kind of chip is compact, integrated, and miniaturized for the Raman signal detection. Furthermore, it can be fabricated easily in large quantities at cost, rendering promising applications.

A Review of Current Methods in Microfluidic Device Fabrication and Future Commercialization Prospects

Microfluidic devices currently play an important role in many biological, chemical, and engineering applications, and there are many ways to fabricate the necessary channel and feature dimensions. In this review, we provide an overview of microfabrication techniques that are relevant to both research and commercial use. A special emphasis on both the most practical and the recently developed methods for microfluidic device fabrication is applied, and it leads us to specifically address laminate, molding, 3D printing, and high resolution nanofabrication techniques. The methods are compared for their relative costs and benefits, with special attention paid to the commercialization prospects of the various technologies.

DERS substrate based on NERS-SERS interaction in integrated microfluidic detection

The systematic simulation study of a structure with nanogap-enhanced Raman scattering and surface-enhanced Raman scattering (NERS-SERS) substrate is presented. This double-enhanced Raman scattering (DERS) substrate with coupling between the localized surface plasmons of noble metal nanosphere colloids and surface plasmon polaritons of a 1D sinusoidal noble metal nanograting is analyzed. With the excitation light wavelength at 785 nm, the key structure parameters of noble metal nanospheres and sinusoidal noble metal nanogratings are deduced by FDTD. With the optimal DERS substrate, the SERS enhancement factor (EF) can be 9 orders of magnitude as possible. The DERS substrate was fabricated, and an extra SERS effect was demonstrated by experiments. This DERS substrate will be integrated with microfluidics in the next work, with the purpose of in situ, real-time, continuous detection of trace water soluble gas-phase or airborne agents, such as trace explosives in air.

In Situ Two-Step Photoreduced SERS Materials for On-Chip Single-Molecule Spectroscopy with High Reproducibility

A method is developed to synthesize surface-enhanced Raman scattering (SERS) materials capable of single-molecule detection, integrated with a microfluidic system. Using a focused laser, silver nanoparticle aggregates as SERS monitors are fabricated in a microfluidic channel through photochemical reduction. After washing out the monitor, the aggregates are irradiated again by the same laser. This key step leads to full reduction of the residual reactants, which generates numerous small silver nanoparticles on the former nanoaggregates. Consequently, the enhancement ability of the SERS monitor is greatly boosted due to the emergence of new "hot spots." At the same time, the influence of the notorious "memory effect" in microfluidics is substantially suppressed due to the depletion of surface residues. Taking these advantages, two-step photoreduced SERS materials are able to detect different types of molecules with the concentration down to 10^-13 m. Based on a well-accepted bianalyte approach, it is proved that the detection limit reaches the single-molecule level. From a practical point of view, the detection reproducibility at different probing concentrations is also investigated. It is found that the effective single-molecule SERS measurements can be raised up to ≈50%. This microfluidic SERS with high reproducibility and ultrasensitivity will find promising applications in on-chip single-molecule spectroscopy.

Surface-enhanced Raman spectroscopy and microfluidic platforms: challenges, solutions and potential applications

The review provides an overview of the development in the field of surface-enhanced Raman spectroscopy combined with microfluidic platforms.

Highly uniform SERS-active microchannel on hydrophobic PDMS: a balance of high reproducibility and sensitivity for detection of proteins

SERS-active microchannels on hydrophobic polydimethylsiloxane (PDMS) with both high reproducibility and sensitivity are fabricated. Proteins might denature while drying on the SERS-active substrate, but keep native structures in the microchannels.

Surface-enhanced Raman scattering substrate based on cysteamine-modified gold nanoparticle aggregation for highly sensitive pentachlorophenol detection

A new surface-enhanced Raman scattering (SERS) substrate based on cysteamine-modified gold nanoparticles (AuNPs) on the glass surface has been developed for the monitoring of pentachlorophenol (PCP) in water samples.

Complete separation of macroscopic rod-like bimetallic nanoassembly perpendicular and parallel on substrate for simultaneous sensing of microorganisms

Although two kinds of macroscopic ordered tridimensional nanoassemblies, i.e., alignment of nanorods, can be yielded by controllable droplet evaporation methods, complete separation of the nanoassembly perpendicular or parallel to substrate is quite challenging. It can, however, be realized by the aid of facet blocking combined with the tuning of ionic strength and colloidal concentration. The as-fabricated rod-like bimetallic nanoassembly has proved to be an excellent SERS active substrate compared to random aggregates. It should be mentioned that macroscopic ordered tridimensional nanoassembly perpendicular to the substrate can be used as a highly active SERS substrate with good uniformity and can be successfully applied for finely discriminating two microorganisms: Escherichia coli bacteria and Saccharomycetes.